Process optimization and crystallization kinetics for preparation of nesquehonite by impinging stream-coprecipitation technology
Impinging stream is a technique to enhance heat and mass transfer in process engineering.Due to the characteristics of its high and uniform supersaturation,it is widely used in powder preparation.Nesquehonite(MgCO3·3H2O)can be used to combine with materials and increase the strength and toughness of materials because of its special rod-like structure.In this work,MgCO3·3H2O crystal was prepared from magnesium chloride and sodium carbonate by a new impinging stream technology combined with co-precipitation method,and the preparation process conditions and crystallization kinetics of MgCO3·3H2O were studied.The effects of reactant concentration c,reaction temperature T,cyclic impact flow rate Q and cyclic impact time t on the crystal morphology and structure of MgCO3·3H2O were discussed by taking the aspect ratio of rod-like MgCO3·3H2O as the index.The optimal conditions were as follows:the reactant concentration was 0.25 mol/L,the reaction temperature was 50 ℃,the cyclic impact flow rate was 500 L/h,and the cyclic impact time was 50 min.Under these conditions,the MgCO3·3H2O crystal with the aspect ratio of 20 and an average crystal length of 57.3 μm can be obtained.The particle size distribution data under different conditions were measured by laser particle size meter,and the crystal nucleus number density,growth rate and nucleation rate of the product were analyzed according to the population balance model.Under the optimal conditions,the nucleation rate of the crystals was 2.061×106 #/(mL·min)and the growth rate was 0.148 μm/min.The nucleation and growth kinetic equations of each factor in the system of impinging stream-coprecipitation reaction were obtained,and the sensitivity coefficient i of each factor on crystal growth was determined.iQ＜iT＜0＜it＜ic is the relationship of sensitivity coefficients obtained by each influencing factor.Cyclic impact flow rate and reaction temperature can promote crystal growth.
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